1. Field of Use
This invention relates to a seat suspension having a compression spring system for resiliently supporting a seat on a vehicle and more specifically to an improved mounting arrangement for the compression springs.
2. Description of the Prior Art
It is known in the prior art to provide a seat suspension having a base part mountable on a vehicle, a seat support part on which the seat is mounted, a parallel link mechanism interconnecting the base and support parts to permit relative vertical movement therebetween, and cylindrical compression spring means mounted between the base part and the seat support part or a part of the parallel linkage. In such known designs, the free ends of the compression spring means are each fully anchored or supported about the entire circumference of their respective free ends by spring support seats usually created by lancing and upsetting a steel plate to form an annular flange that projects into the internal bore of the spring. U.S. Pat. No. 3,917,209, issued Nov. 4, 1975 to Albert John Adams, discloses a typical suspension system of this type showing spring seats fully anchoring both ends of the spring.
The axis of an unstressed compression spring lies in a straight line or if the spring is lightly compressed, the axis may be slightly bowed depending upon the wire gauge of the spring. For maximum spring life it has long been known that the direction of a force vector indicating the force applied when sitting on the seat to stress the compression spring during vertical floating of the seat should be parallel to the spring's straight line axis to maintain the axis as close to straight line as possible. As a practical matter this is not achieved. The ends of the parallel links must follow an arcuate path and the seat support part will shift vertically and, to some minor extent, horizontally fore and aft relative to the fixed base part, thus forcing the axis of the spring to shift and modify its shape during vertical floating of the seat. However, as the ends of the spring are fully anchored, horizontal fore-to-aft shifting of the seat support part relative to the base requires that the spring axis fluctuate and, at some point, modify its straight line axis into a non-straight line axis having an ogee or S curve configuration. Exactly when an S curve will occur depends upon the specific geometry of a given suspension system and preferably it will occur, for example, as the suspension reaches its vertically uppermost position.
Causing the spring axis to so modify fatigues the spring and reduces its life very significantly. Slightly bowing the spring axis does not significantly shorten spring life. The forming of S curves causes a larger number of sharper bends which stress the spring and the usual result is that the spring prematurely catastrophically breaks. Spring breakage has several adverse results. First, breakage can project spring pieces into the surrounding area with injury causing force. Second, spring breakage allows the seat to abruptly bottom out which can cause the operator to lose control of the vehicle. Third, the spring must be replaced, which may be very difficult and expensive when the suspension assembly is welded and riveted together. Frequently the spring cannot be conveniently replaced and then the suspension arrangement must be replaced as a unit at very significant cost which generates customer dissatisfaction based on a belief that he has received a defective product.
To minimize such fluctuation and modification of the spring axis shape, two design principles are usually followed in the prior art. First, it is common practice to make the parallel links as long as space limitations will allow. This is done to permit the compression spring to be made as long as possible so that it will not assume an S shape. Second, the links are orientated to extend more closely to the horizontal rather than the vertical so that the ends of the links connected to the seat support part will follow an arcuate path having a large radius orientated to be substantially vertical and thereby minimize both the amount of fore-to-aft travel and modification of the spring axis shape. This requires the seat support links and springs to be unduly large and heavy, with a high overall profile, all of which increases the cost of manufacture.
Orientating the parallel links more closely to horizontal also leads to certain functional disadvantages. To facilitate the operator's dismounting, the seat should be moved aft, that is, away from the steering wheel a substantial distance in order to provide the operator with a longer fore-to-aft access space to more easily accommodate passage of the leg and thigh as he gets off of the vehicle. Preferably this aft seat movement should occur automatically, in immediate response to removal of the operator's weight as he starts to rise to dismount from the vehicle. With the use of parallel links that are orientated more closely to horizontal than vertical, just the opposite result occurs. As the operator starts to rise, the absence of his weight allows the seat to rise but the ends of the links move primarily vertical and there is very little fore-to-aft travel. This results in the forward edge of the seat moving upward and thus closer to the steering wheel. Therefore in known designs the seat suspension actually can function to reduce the space available to accommodate the operator's leg and thigh during dismounting. The solution to this problem has been to mount the seat suspension on horizontal tracks for selectable fore-to-aft movement. In theory, this allows the operator to always provide space necessary for safe dismounting but in practice there are two major drawbacks to this design. First, the operator must take positive action to actually cause the seat to move aft before dismounting. However, frequently the operator is in too much of a hurry to leave the seat, with the result that he or she does not slide it back which increases the risk of tripping or falling during dismounting. Second, the track mechanism materially increases the complexity of the seat suspension and the cost of manufacture.
What is needed therefore is a seat suspension of simple design with a minimal number of parts that will permit the seat immediately and automatically to travel substantial distances in both fore-to-aft and vertical directions when the operator starts to dismount to minimize the need for an independent horizontal track mechanism, and that will also enable the compression spring axis to automatically shift without modification of its shape during floating, thus allowing its overall axial length to be maintained at a minimum to facilitate the ability to locate the spring directly below a force vector generated by the weight of the operator on the seat.